Gas-filled electric incandescent lamp



16d States Patent @fii fi 2,891,189 Patented June 16, 1959 GAS-FILLED ELECTRIC [NCANDESCENT LAMP Pal Bud, Istvan Gtics, and Emil Theisz, Budapest, Hungary, assignors to Egyesult Izzolampa es Villamossagi Reszvenylarsasag, Budapest, Hungary Application April 23, 1957, Serial No. 654,546 Claims priority, application Hungary April 28, 1956 3 Claims. Cl. 313-226) This invention relates to gas-filled electric incandescent lamps, containing xenon and being provided with a coiled metal filament, especially to lamps with a high luminous efiiciency intended for special purposes and constructed to be used at low voltages, i.e. on voltages not surpassing 40 volts.

Electric incandescent lamps containing xenon and having a high luminous efficiency are known in the art, it being also known that their luminous efliciency, expressed in lumen watt and calculated for an identical life, surpasses that of the lamps of vacuum type, respectively of lamps filled with argon, by about 20 respectively 30 percent. This very favourable luminous efiiciency, however, cannot be. satisfactorily utilized in actual working practice owing to the fact that a substantial percentage of such lamps cannot reach its rated life for which it was constructed and designed, because itsfilament burns out sooner. The reason of this undue shortening of the rated life of such lamps is that flash-over takes place along the filament. Owing to this reason lamps filled with pure or substantially pure xenon have not been used up to now in actual industrial practice, with the exception of lamps constructed for voltages of some volts below the ionisation voltage of xenon, in actual practice, lamps for voltages under 10 volts.

In the case of lamps constructed for higher, especially mains, voltages, various means have been suggested and are known for lessening the danger of flash-over. The most usual of these means, already described in Patent No. 2,060,657, consists in the admixture of a substantial quantity of neutral gases, usually nitrogen, in quantities exceeding 5%, to the rare gas, i.e. krypton and/ or xenon. This admixture of nitrogen lessens the danger of flashover in case of lamps containing rare gases such as kryp ton, Xenon or both, but deteriorates their luminous efficiency. Its use, however, has been imperatively necessary in case of lamps of a rated voltage surpassing volts and therefore lamps containing krypton and/or xenon have always been provided in actual practice with a substantial admixture of nitrogen.

In order to avoid deteriorating the favourable luminous efliciency of lamps filled with xenon which deterioration was caused by the admixture of nitrogen, it has been already suggested and tried to lessen the added quantity of nitrogen and to avoid the danger of flash-over by other means than that of admixing nitrogen to the xenon, which if used alone, ensures an improvement of the luminous efiiciency of about 20 to 30%, as stated above. In case of lamps filled with krypton it has been snggestedto use various oxides, for example silicium dioxide or titanium dioxide, usually employed as coatings provided on the inside wall of the bulb, as these compounds are adapted to lessen the danger of flash-over by binding the alkaline impurities present in the bulb. Simultaneously it has been tried to manufacture certain inner parts of the lamp, such as the lead-in conductors supporting the filament, of a material of high work-function. A drawback of the use of the oxides mentioned above consists in the fact that owing to the oxide coating present on the inner surface of the bulb the lamp has the character of a lamp with a frosted or opal bulb, this not being desirable in the majority of cases and moreover causing a deterioration of the luminous efficiency by the absorption of light effected by said coating. Manufacturing the conductors holding the filament of special materials is expensive and has drawbacks also from other points of view.

Accordingly, it is an object of the invention to provide a lamp containing xenon as a filling gas and a coiled metal filament, constructed for rated voltages below 40' volts and especially above about 10 volts, in which the improvement ensured by the use of xenon can be actually utilized in working practice, owing to the lessening of the danger of flash-over and thereby ensuring a useful life of the lamp substantially corresponding to that which it has been constructed for, Le. to its rated life.

In the present specification and claims, the expression xenon is intended to designate not only pure or at least technically purexenon gas, but also gas mixtures consisting of rare gases and containing xenon as an overwhelming constituent, i.e. at least about 80% by volume of xenon, while the rest may consist of krypton, argon or a mixture of these. Such mixtures of these rare gases present, from the point of view of the danger of flashover, approximately the same characteristics as pure xenon, and do not deteriorate the advantageous improvement of luminous efliciency obtainable by the use of pure xenon to a practically objectionable extent. Nevertheless, it is preferable if the gas mixture used as the filling :gas of the lamps according to the invention contains as much xenon as possible, as the admixture of. other rare and/or neutral gases of smaller molecular weight always causes some lessening of the luminous efficiency of the lamps, according to the well-known teachings of the specification of the Patent No. 2,060,657 already mentioned above. The high price of pure or technically pure xenon, amounting to a multiple of the price of the usual mixture of krypton and xenon, used for filling lamps with heavy rare gases in the majority of cases, and containing xenon only in about the same proportion to krypton as the atmospheric air, does not constitute an economical impediment to the use of the lamps according to the invention. This is due to the fact that the special lamps according to the invention oflfer such advantages that a higher price is justified by these, and that the bulb volume of special lamps of rated voltages below 40 volts being relatively small, only small quantities of the expensive xenon are needed for filling them.

It is another object of the invention to ensure the advantages stated above without employing expensive materials as supports of the filament and/or light-absorbing oxide coatings inside the bulb of the lamp.

The invention is based on our discovery that the danger of flash-over in lamps containing xenon may be practically obviated in lamps burning at voltages below 40 volts by a suitable construction of the lamps, in conjunction with the use of a small percentage of nitrogen substantially below that which has been. found to be necessary up to now.

In the course of our experiments we have ascertained that the danger of flash-over is the maximum between those points of the filament and/or its support, between which points the differenceof electrical potential, i.e., the voltage, is the maximum, and therefore the distance between these points is critical in this respect. Further experiments have resulted in ascertaining that if this distance is chosen at or above the value ascertained on grounds of the rated voltage, the rated energy consumption and the luminous eificiency of the lamps by means of an equation given below, and the percentage of nitrogen chosen to a value not below that which has been ascertained by another equation, also stated below, the danger of flash-over is practically absent. In consequence of this fact, such lamps can be used in actual practice during all the time of their rated life, and ensure, owing to the small percentage of nitrogen admixed to the xenon, a very high luminous eificiency almost equal to that which can be obtained by using xenon as a filling gas without the admixture of nitrogen. This very favourable result may be achieved at the pressures of the xenon usual in lamps up to now, i.e. between 450 and 700 Torr (millimetres of mercury column).

Accordingly, the electric incandescent lamp according to our invention comprises, in combination, a filling of xenon, a coiled metallic filament, usually tungsten filament, the rated voltage of the lamp being below 40 volts and the pressure (at room temperature) of the gas mixture filling the bulb being between 450 and 700 Torr, and the percentage of nitrogen in the gas mixture of the lamp being not below the value given by the equation wherein N designates the percentage by volume of nitrogen, V the rated voltage of the lamp and K a numerical factor, the value of which is between 3 and 4, depending on the pressure of the gas in the lamp, the values-of 3 resp. 4, being valid for pressures of 450 resp. 7 Torr, and a filament and its supports disposed in such a manner, that the minimum distance t between the nearest points of highest potential difference of the filament, respectively of the filament and one of its supports isnot below the value given by the equation wherein t designates said minimum distance in millimetres, V the rated voltage of the lamp, W the rated energy consumption of the lamp, burning at its rated voltage, in watts, and L/W the luminous efficiency of the lamp expressed in lumen/watt, measured at the rated voltage of the lamp and C a numerical factor, the value of which ranges, according to the construction and destination of thelamp, between 0.015 and 0.035. In this connection, it is to be noted that the highest potential difference referred toabove is substantially equal to the rated voltage of the-lamp and that the distance t given by the equation may be surpassed to any practical extent without impairing thereby the lamp, whereas the value of the percentage of nitrogen should be kept, within the limits of practical manufacturing tolerances, at the value given by the equation, as surpassing this value deteriorates the luminous efficiency of the lamp, as is well known to those skilled in the art. 7

The invention will be more particularly detailed hereinafter in connection with the drawing of this specification.

On the drawing:

Figure 1 shows anembodiment of the lamp in a side elevational view, with part of the bulb broken away, resp. in section, for the sake of better illustration.

Figure 2 shows in a diagrammatical side elevational view, the' arrangement of the lead-in conductors and the filament of another lamp, and

Figure 3 shows in a diagrammatical side elevational view, the arrangement of the filament and screens of a special lamp, such as a lamp for a motor-car headlight.

The structure of the lamp shown by Figure 1 is the conventional one; the glass bulb I of the lamp being fastened to thecap' 2 in the usual manner, and containing as a filling gas, a mixture of xenon and nitrogen. The lead-in conductors '3 and 4 support the coiled tungsten filament 5 disposed in a practically straight line between its points of connection with the conductors 3 and 4. It has been ascertained by our-experiments that the points of the filament between which flash-over is most liable to oc'cure are those points of the coiled part of the filament, between which the voltage difference is maximum, i.e. the points where the coiled part of the filament begins and ends. The distance between these points is designated on all the figures of the drawing by t. From the comparison of Figures 1 and 2 it is obvious, that thisdistance is, for identical lengths of coiled filament parts, shorter in case of the semicircularly arranged filament 6 of Figure 2 than for the straight filament 5 of Figure 1, the filaments being in both cases disposed substantially in a plane but along a straight line in Figure 1, and a curved line in Figure 2. The potential difference between the critical points at the ends of the distance it substantially corresponds to the rated voltage of the lamp, as the voltage drop taking place along the lead-in conductors and the short uncoiled filament parts is insignificant, amounting at most to about 1-5 The lamp shown by Figure--1 is constructed for a rated voltage of 20 volts, and an energy consumption of 20 watts at this voltage, and has a luminous output of 340 lumens. Its-bulb 1 contains xenon with an admixture of nitrogen at a pressure of 690 Torr. According to the equations stated above, the value of the distance t amounts to 9.3 millimetres, and the percentage of nitrogen to 3.75%. It may be seen clearly from the drawing, that while it is comparatively easy to construct the lamp withthe straight filament arrangement shown "on Figure l, in a manner to ensure said minimum distance of 9.3 mm. between the points critical as regards-flash-over, care has to be taken to ensure this in thecase of the semicircularly disposed filament of Figure 2.

The constructive rule provided by this invention is of special importance incase oftl amps having a light-reflecting metallic shield disposed close'to the filament, anfexample of the arrangement of the parts of such-a lamp ,part fi connected to the other end of the coil and acting as its support, whereas the other support 9 of the filament, although very'close to the support 8 on several places, presents no points dangerous as regards to flashover.

lt has'ito be' pointed out that the lamps according to v tam-ma be constructed for various special purposes, especially as lamps of small size, such as are used in: railway vehicles, motor-cars, bicycles, motorcycles, miners lamps, sound-track illuminating lamps for-movie films, etc., and a substantial improvement of such special lamps of low rated voltages may be achieved by: means of this invention,

;It isv therefore to be understood that the embodiments ojfthis invention herein shown and described are to be taken only as examples of the same, and that various changes in the -structure of the lamps may be resorted to the percentage of nitrogen contained in the gas mixture filling the bulb substantially corresponding to the value given by the equation wherein N designates the percentage by volume of nitrogen, V therated voltage of the lamp, and K a numerical factor of a value ranging between 3 and 4, said filament and its said supporting conductors being arranged so as to ensure that the minimum distance r between the nearest points of highest potential difference of the filament, respectively of the filament and one of its supports is not below the value given by the equation wherein t designates said minimum distance in millimetres, V the rated voltage of the lamp, W the rated energy consumption of the lamp, burning at its rated voltage, in watts, and L/ W the luminous efficiency of the lamp expressed in lumen/watt, and C a numerical factor of a value ranging between 0.015 and 0.035.

2. An electric incandescent lamp of a rated voltage below 40 volts, but above volts, comprising, in combination, a vitreous bulb filled with a gas mixture containing xenon and nitrogen at a pressure ranging between 450 and 700 Torr, a coiled metallic filament supported by metallic lead-in conductors inside said bulb, the percentage of nitrogen contained in the gas mixture filling the bulb substantially corresponding to the value given by the equation wherein N designates the percentage by volume of nitrogen, V the rated voltage of the lamp, and K a numerical factor of a value ranging between 3 and 4, said filament and its said supporting conductors being arranged so as to ensure that the minimum distance t between the nearest points of highest potential diiference of the filament, respectively of the filament and one of its supports is not below the value given by the equation T=V.W.L/W.C, wherein t designates said minimum distance in millimetres, V the rated voltage of the lamp, W the rated energy consumption of the lamp, burning at its rated voltage, in watts, and L/W the luminous efiiciency of the lamp expressed in lumen/watt and C a numerical factor of a value ranging between 0.015 and 0.35 and wherein said coiled filament is disposed substantially in a plane and said distance t lies between the first and last coil of said filament.

3. An electric incandescent lamp of a rated voltage below 40 volts, but above 10 volts, comprising, in combination, a vitreous bulb filled with a gas mixture containing xenon and nitrogen at a pressure ranging between 450 and 700 Torr, a coiled metallic filament supported by metallic lead-in conductors inside said bulb, the percentage of nitrogen contained in the gas mixture filling the bulb substantially corresponding to the value given by the equation wherein N designates the percentage by volume of nitrogen, V the rated voltage of the lamp, and K a numerical factor of a value ranging between 3 and 4, said filament and its said supporting conductors being arranged so as to ensure that the minimum distance 1 between the nearest points of highest potential difierence of the filament, respectively of the filament and one of its supports is not below the value given by the equation t=V. /I7. /L/ W.C, wherein t designates said minimum distance in millimetres, V the rated voltage of the lamp, W the rated energy consumption of the lamp, burning at its rated voltage, in watts, and L/W the luminous efficiency of the lamp expressed in lumen/watt, and C a numerical factor of a value ranging between 0.015 and 0.035 and wherein said filament disposed substantially in a plane is electrically connected to a metallic shield disposed inside said bulb and said distance t lies between a coil of said filament and a metallic part electrically connected with said filament.

References Cited in the file of this patent UNITED STATES PATENTS 2,115,480 Claude Apr. 26, 1938 

1. AN ELECTRIC INCANDESCENT LAMP OF A RATED VOLTAGE BELOW 40 VOLTS, BUT ABOVE 10 VOLTS, COMPRISING, IN COMBINATION, A VITREOUS BULB FILLED WITH A GAS MIXTURE CONTAINING XENON AND NITROGEN AT A PRESSURE RANGING BETWEEN 450 AND 700 TORR, A COILED METALLIC FILAMENT SUPPORTED BY METALLIC LEAD-IN CONDUCTORS INSIDE SAID BULB, THE PERCENTAGE OF NITROGEN CONTAINED IN THE GAS MIXTURE FILLING THE BULB SUBSTANTIALLY CORRESPONDING TO THE VALVE GIVEN BY THE EQUATION 